fix: free retired learner's email on retirement completion

[rahulkanneri-2u]

Description

Retirement rewrites auth_user.email to a deterministic hash and keeps that row forever for compliance, but is_email_retired() matches that hash permanently, so a retired learner could never register again with their own former email.

This adds free_retired_learner_email(user) (accounts/utils.py), which appends .freed.<user_id> to the retired-hash email once retirement is COMPLETE. That's enough to stop it matching is_email_retired(), without deleting the row or nulling the email (both break compliance/uniqueness constraints). It's a no-op if the email's already been freed.

The handler behind PATCH /api/user/v1/accounts/update_retirement_status/ that tubular calls on every state transition so the email is freed automatically the moment retirement reaches COMPLETE.

A free_retired_user_email management command (--username/--user_id) is also added, for backfilling learners retired before this existed.

Impact: Learners can re-register with a previously-retired email once retirement completes.
Operators get a command to backfill older retirements. No UI changes, no migration.

Testing instructions

• pytest openedx/core/djangoapps/user_api/accounts/tests/test_utils.py -k FreeRetiredLearnerEmail
• pytest openedx/core/djangoapps/user_api/accounts/tests/test_retirement_views.py -k TestAccountRetirementUpdate
• pytest openedx/core/djangoapps/user_api/management/tests/test_free_retired_user_email.py

Other information

• No database migration; only mutates the email column on already-retired rows the UserRetirementStatus audit row is untouched.

[vgulati-apphelix]

Suggested change

	parser.add_argument('--username', type=str, help='Username of the retired learner to free.')
	parser.add_argument('--user_id', type=int, help='User ID of the retired learner to free.')
	user_group = parser.add_mutually_exclusive_group(required=True)
	user_group.add_argument('--username', type=str, help='Username of the retired learner to allow email reuse.',)
	user_group.add_argument('--user-id', type=int, help='User ID of the retired learner to allow email reuse.',)

[vgulati-apphelix]

Logger to be sanitized - remove email & username.

[vgulati-apphelix]

Suggested change

	logger.info(f'Successfully freed email for user {user.id}.')
	print(f'Successfully freed email for user {user.id}.')
	logger.info(f'Successfully freed email for user {user.id}.')